offering, sacrifice
贄 centers on a formal gift given to a higher power or authority: an offering or sacrifice, often with a sense of ritual presentation.
贄 combines 執 (hold, take) with 貝 (shell, money, valuables), suggesting holding up valuables as an offering. The exact historical development is uncertain, but the component relationship is clear.
The top part 執 means to hold or grasp, and the bottom 貝 represents valuables or money. Picture someone holding up a precious shell as a ritual offering or sacrifice.
For シ, imagine a priestess (シ) holding up a shell offering: the priestess's title cues シ while she presents the sacrifice.